WO-02/45 096 describes a fuel rod of the above-mentioned type. This document describes, in particular, the use of chromium oxide Cr2O3 as an additive in fuel pellets in order to promote the thermal creep thereof and limit the risks of the cladding becoming damaged owing to the phenomenon of pellet cladding interaction (PCI).
The cladding of the rod is the first barrier of containment for fission products, the other barriers being constituted by the vessel of the reactor and the concrete chamber thereof.
During normal operation, (situations referred to as class 1), and incident operation (situations referred to as class 2), the tightness of the cladding with respect to the fission products must therefore be ensured.
During a transient power occurrence which corresponds to a class 2 situation, the power reached locally in the fuel can be two to three times greater than nominal power. This rapid increase of power brings about significant expansion of the pellets. Since the thermal expansion of the pellets is greater than that of the cladding, the cladding is consequently placed in a state of traction by the pellets and the stresses at the inner surface of the cladding are increased. These stresses progressively relax by means of creeping. Furthermore, this mechanical stress takes place in a harsh chemical environment owing to the fission products, such as iodine, released by the fuel during the transient power occurrence.
This is referred to as Pellet Cladding Interaction, a phenomenon which can lead to the fracture of the cladding.
Such a fracture of the cladding is not permissible for safety reasons, since it could lead to fission products being released into the coolant system of the reactor.
As shown by the vast majority of tests on fuel rods which have been fractured by PCI in test reactors, the risk of fracture is undeniably localized: radially (at the inner surface of the cladding), axially (in the inter-pellet planes) and azimuthally (opposite the primary radial cracks of the fuel pellets).
At high power, the difference in diametral displacement between the fuel and the cladding, and the diametral over-displacement of the pellet, are exacerbated at the ends of the pellet (deformation of the pellet brought about by the radial thermal gradient in the fuel). There is consequently a high level of stress at the inner surface of the cladding, which level may exceed the elastic limit of the material which constitutes the cladding, generally Zircaloy-4, thus causing damage to the cladding.
This mechanical load is even greater in the region of the inter-pellet planes and at the points of contact between the cladding and the edge of the primary radial cracks brought about by the fragmentation of the fuel pellets during irradiation under normal operating conditions. Furthermore, the high temperature level in the pellets promotes the release of fission products, such as iodine, which condense at the inner surface of the cladding, preferably in the region of the inter-pellet planes (zones which are less hot) and opposite the primary radial cracks of the fuel (preferred path for the discharge of the fission gases).
When the cladding fractures owing to PCI, the cladding becomes cracked and the fission gases may contaminate the coolant system.
The use of Cr2O3 as a doping additive in fuel pellets which are loaded in claddings of Zircaloy-4 has been found to be advantageous with regard to PCI.
However, the risks of fracture owing to PCI are still not reduced to a large enough extent, with the result that the level of operational flexibility of reactors using rods of this type remains too low.